1. Field of the Invention
The present invention relates to a dry etching system and a dry etching method and more particularly, to a dry etching system and a dry etching method using plasma, which are used for semiconductor device fabrication.
2. Description of the Prior Art
In recent years, with the progressing integration degree of semiconductor integrated circuit devices, dry etching systems using plasma have been broadly used, because they are capable of anisotropic etching.
In the dry etching systems of this sort, plasma is generated in a reaction chamber. The plasma typically contains active species such as neutral radicals and positively charged ions. The neutral radicals and the positive ions cooperatively serve to generate an etching action. The neutral radicals react with an "etch object", which is an object to be etched. They play a main role of the etching action. The ions give their energy to the exposed surface of the etch object by ion bombardment. The etching action generated by the radicals is assisted by the energy thus given by the ion bombardment.
To enhance the anisotropy of the etching, it is required for the positive ions to arrive on the surface of the etch object at an approximately right angle with respect to the surface of the object. In other words, the incident angle of the ions needs to have a value as close as possible to a right angle. However, it is not easy to realize that the incident angle of the ions has a value approximately equal to a right angle. This is because the ions are scattered by the collisions with the neutral molecules of a reaction gas introduced into the reaction chamber before they are contacted with the object.
If the pressure of the reaction gas in the reaction chamber is lowered to decrease the number of the neutral gas molecules and the probability of collision of the ions with the neutral gas molecules, the moving or traveling directions of the ions can be aligned to considerable extent. However, in this case, there arises a problem that the density of plasma itself is reduced.
So, improved dry etching systems were developed, which make it possible that an obtainable plasma density is satisfactorily high when the pressure of the reaction gas in the reaction chamber is low enough to align the moving directions of the ions. An example of the improved systems was disclosed in the Japanese Non-Examined Patent Publication No. 6-84811 published in March 1994.
FIG. 1 schematically shows the structure of an example of the conventional improved systems.
In this dry etching system shown in FIG. 1, a dielectric plate 105 is fixed to the top of a reaction chamber 101 electrically connected to the grounded. In the outside of the reaction chamber 101, a radio frequency (rf) induction coil 102 is fixed onto the dielectric plate 105. The coil 102 is a flat, spiral coil, which is formed on a flat plate. Both ends of conducting wires of the coil 102 are electrically connected to a rf power supply 103 arranged outside the reaction chamber 101.
A bottom electrode 109 is fixed to the bottom of the reaction chamber 101. The bottom electrode 109 is electrically connected to a rf power supply 104 arranged outside the reaction chamber 101. The top of the electrode 109 is located in the inside of the reaction chamber 101. A semiconductor wafer 108 is placed on the top of the electrode 109 as the etch object on operation.
A shield 113 is fixed to the chamber 101 in the reaction chamber 101. This shield 113 serves to protect the areas of the wafer 108 from the active etching species.
In the conventional dry etching system shown in FIG. 1, on operation, the reaction chamber 101 is exhausted to a fixed degree of vacuum and then, a specific reaction gas is introduced into the chamber 101 to represent a specific pressure. An rf electric current is supplied to the induction coil 102 from a rf power supply 103. Due to the electric current flowing through the coil 102, a rf magnetic-field (not illustrated) is induced within the reaction chamber 101 through the dielectric plate 105. This magnetic field is almost parallel to the plate 105. Because of the induced magnetic field, plasma (not illustrated) with a high density is able to be generated within the reaction chamber 101 even if the reaction gas in the chamber 101 has a low pressure.
On the bottom electrode 109 located to be opposite to the dielectric plate 105, a semiconductor wafer 108 is located as the etch object. When a rf voltage is applied to the electrode 109, a self-bias voltage will occur on the electrode 109. Due to this self-bias voltage, the ions contained in the plasma are able to arrive on the wafer 108 located on the electrode 109 along a direction almost perpendicular to the wafer 108. Thus, a highly anisotropic etching can be realized.
Generally, with the plasma etching or plasma-assisted etching, the etching action is generated by the chemical reactions of the neutral radicals with the etching object and by the ion-assisting effect of the ions due to the ion bombardment. It is effective to prevent the side etching of the etching object utilizing the "sidewall protection layer" for the purpose of etching the object more anisotropically with high preciseness.
The "sidewall protection layer" is formed during an etching process in such a way that so-called "reaction products" are deposited on the side faces of the remaining, etched object (i.e., unetched part) and of an overlying masking layer. The "reaction products" are generated by the chemical reactions of the sputtered materials from the object with the reaction gas in the reaction chamber 101.
It is needless to say that the reaction products are deposited on any areas other than the side faces of the unetched object. For example, the products are deposited on the inner surface of the dielectric plate 105, the exposed surface of a resist mask formed on the object, and the exposed surface of the object.
However, since the surface of the resist mask and the exposed surface of the etch object are almost perpendicular to the etching direction, no products tend to grow thereon. Specifically, on these surfaces, the etch rate of the object is greater than the growth rate thereof due to the effect of the ion bombardment and therefore, the reaction products will not be deposited thereon. As a result, the reaction products grows only on the side faces of the unetched object and the resist mask formed on the object, thereby selectively forming the sidewall protection layer.
The inner surface of the dielectric plate 105 is not affected by the ion bombardment. Therefore, the reaction products will grow on the inner surface of the plate 105 in the same way as those of the remaining, unetched object and the resist mask.
With the dry etching system shown by FIG. 1, the reaction products deposited on the inner surface of the dielectric plate 105 tend to be sputtered by the etching species, thereby emitting the reaction products in a gas phase into the chamber 101 again. The reaction products will be redeposited on the side faces of the remaining, unetched object. This redeposited products will affect the edge profile of the etch object, resulting in a problem that a wanted edge profile is not achieved.
This problem becomes conspicuous when a plurality of the semiconductor wafers 108 are successively etched by using the conventional dry etching system of FIG. 1. Specifically, in this case, the quantity of the vapor-phase reaction products existing in the reaction chamber 101 increases with the increasing number of the semiconductor wafers 108, thereby raising the quantity of the redeposited reaction products on the surface of the etch object. As a result, with increase of handling number of sheets, there is a problem that the edge shape of the etch object deviates from the wanted or designed one as the number of the etched wafers 108 increases.